Personal radiation dosimeters

ABSTRACT

A personal radiation dosemeter comprising a radiation detector means coupled to an electronic measurement circuit arranged in combination therewith to generate signals representative of an amount of radiation detected by the radiation detector. The radiation detector means is arranged to be screened from ambient light and is further provided with a light source optically coupled to the radiation detector and arranged to operate under control of a test control circuit to generate light of a wavelength which may be detected by said radiation detector. This arrangement provides, in combination with said electronic measurement circuit an integrity test for said radiation detector means.

[0001] The present invention relates to personal radiation dosemeters orindicators which operate to measure radioactivity doses to whichpersonnel are exposed.

[0002] Electronic personal dosemeters are worn by personnel inpotentially hazardous environments. An example of such an environment isa nuclear power station where there is the possibility, however small,of personnel receiving radiation doses which may be potentiallyhazardous to their health. For this reason, personnel are often requiredto carry or wear an electronic personal dosemeter which operates todetect radiation and provide an indication of an amount of radiation towhich the personnel are exposed at any time.

[0003] Electronic personal dosemeters are provided with radiationdetectors which operate to generate a signal for each particle ofradiation detected. Electronic personal dosemeters may furthermoreprovided with a means for converting signals generated or detected bythe radiation detector into an audio signal so that a person we thedosemeter is given an audible indication of a current level of radiationto which their body is being exposed.

[0004] Typically, during a day's activity, a person working in a powerstation may be in an environment where there are relatively largeamounts of radiation during one part of the day, and in an environmentin which there is virtually no radiation, or only background radiationpresent, in another part of the day. As such, a technical problem existsin that, if the electronic personal detector worn by a person shouldmalfunction at a time when the person is in an environment where thereis no radiation, then the person wearing the detector will not bealerted to the presence of harmful amounts of radiation as a result ofthe malfunction of the dosemeter. A technical problem therefore existsin arranging for the radiation dosemeter to be tested in a way whichprovides regular confirmation of the integrity of the radiationdosemeter.

[0005] The technical problem of testing and confirming the integrity ofa radiation dosemeter is addressed by the present invention.

[0006] According to the present invention, there is provided a personalradiation dosemeter comprising a radiation detector means coupled to anelectronic measurement circuit arranged in combination therewith togenerate signals representative of an amount of radiation detected bysaid radiation detector, wherein said radiation detector means isprovided with a light source optically coupled to said radiationdetector and arranged to operate under control of a test control circuitto generate light of a wavelength which may be detected by saidradiation detector, thereby providing in combination with saidelectronic measurement circuit an integrity test for said radiationdetector means.

[0007] The term light as used herein refers to light in both the visibleand invisible spectra.

[0008] By arranging for the electronic personal dosemeter to be providedwith a light source capable of generating photons in the visible orinfra-red spectra, and arranging for the test control circuit togenerate or energise the light source at predetermined intervals, theelectronic personal dosemeter is provided with a means for testing theintegrity of the radiation detector and the electronic circuit meansthereby confirming the integrity of the radiation dosemeter.

[0009] The light source may be a light emitting diode. The opticalcoupling may include a fibre optic. The optical coupling may be effectedfrom reflection from a surface of the shield.

[0010] A further problem with known electronic personal dosemeters isthat the radiation detectors are susceptible of providing false readingsas a result of electromagnetic interference. For example, strayelectromagnetic fields generated by computer monitors, radar systems ormobile phones may cause a false reading of a radiation particle to bedetected.

[0011] The radiation detector should also be provided with a screen orshield from certain low energy X-rays or gamma photons in order tofilter such low energy X-rays or gamma photons in order to provide abetter representation of radiation received by the human body. To thisend, the radiation detector must be provided with a radiological shieldas well as a shield from electromagnetic interference. A technicalproblem therefore exists in providing a personal radiation dosemeterwith a shield from both electromagnetic interference and a radiologicalshield. This technical problem is addressed by a first aspect of thepresent invention.

[0012] According to a first aspect of the invention there is provided apersonal radiation dosemeter comprising a radiation detector meanscoupled to an electronic measurement circuit and arranged in combinationto generate signals representative of an amount of radiation received bysaid radiation detector means, wherein there is further provided ashield arranged to be electrically coupled to an earth plane and tosubstantially surround a volume in which said radiation detector meansis disposed, said shield being fabricated from electrically conductivematerial so as to provide substantial electromagnetic screening and saidelectrically conductive material has a composition and density which issufficiently high to provide substantial radiological shielding forsubstantially low energy radiation particles, said shield being therebyarranged to provide both electromagnetic and radiological screening.

[0013] The electrically conductive material may be a metal. The metalmay be tin. The metal may be an alloy such as pewter.

[0014] By fabricating a shield which provides both radiological andelectromagnetic screening to the radiation detector, a reduction in sizeand weight of the radiation dosemeter may be effected. This isparticularly advantageous for personal dosemeters.

[0015] Known radiation dosemeters are arranged to provide an audiblesignal for an integral number of particles of radiation detected by aradiation detector. However, there is a requirement to provideinstruments to detect radiation, with different characteristics. Thisrequires the use of more than one detector, each of which is arranged todetect radiation with different characteristics. A technical problemtherefore exists in providing an audible signal which is representativeof the radiation dose rate received by the human body without dependenceupon the characteristics of the radiation detector.

[0016] Known radiation dose rate meters use a microprocessor to effectprocessing and combination of signals from a plurality of detectors.However, microprocessors consume a significant amount of power whilst inan operating mode and for this reason may be only activated on a basisof a duty cycle. The duty cycle has an effect that battery power iseconomised. However the duty cycle of the microprocessor is such that amaximum repetition frequency of the audio signal indicative of theradiation dose rate may be unacceptably low. These technical problemsare addressed by a second aspect of the present invention.

[0017] According to a second aspect of the present invention there isprovided a personal radiation dosemeter for generating a monitor signalrepresentative of a radiation dose rate, said radiation dosemetercomprising a radiation detector means coupled to an electronicmeasurement circuit arranged in combination therewith to generatesignals representative of an amount of radiation received by saidradiation detector, wherein said electronic measurement circuit includesat least one data store, an accumulator means and control circuit means,which control circuit means is coupled to said radiation detector meansand arranged to add a predetermined number stored in said data store toan accumulated total stored in said accumulator in response to signalsfrom said radiation detector means, said control circuit being arrangedto generate a monitor signal for each increment of said accumulatedtotal by said predetermined numerical threshold, which monitor signal isfed to an audio signal generator so as to provide an audible signal inaccordance with said increment, a repetition frequency of said audiblesignal being thereby representative of said radiation dose.

[0018] By arranging for a control circuit to add a predetermined numberto an accumulator means when a signal is received from the radiationdetector means, a scaling of the signal received or generated by theradiation detector means may be effected by appropriately selecting thenumber stored in the data store and the numerical threshold which isused to trigger the monitor signal when the numerical threshold isreached. In this way scaling of the signal generated by the radiationdetector means is effected without a requirement for a microprocessor,thereby allowing the control circuit to be implemented in hardware thuseffecting a substantial improvement in power consumption of theelectronic personal dosemeter.

[0019] The personal dosemeter may further include at least one otherradiation detector means and at least one other data store wherein saidat least one other data store includes a further predetermined numberand said control circuit operates to add said further predeterminednumber to said accumulator on consequent upon receipt of signals fromsaid at least one other radiation detector means.

[0020] By providing a further radiation detector which may be arrangedto detect a different type or energy of radiation particle and providinga further data store which is arranged to be prestored with a furtherpredetermined number, the control circuit may be arranged to add saidfurther predetermined number when said further radiation detector meansdetects the presence of radiation. In this way the accumulator maintainsa total representative of an amount of radiation detected by saidradiation detector means and said further radiation detector means inproportion to the first and further predetermined numbers. By selectingthe first and the further predetermined numbers independence upon thenumerical threshold, the radiation dosemeter may be arranged to generatea monitor signal at a repetition frequency which is arranged to providean audible indication determined by the relative amounts and harmfuleffect of the radiation detected by the first and further radiationdetector means.

[0021] One embodiment of the present invention will now be described byway of example only with reference to the accompanying drawings wherein:

[0022]FIG. 1 is a schematic block diagram of a personal radiationdosemeter illustrating, in particular, a screen arrangement around theradiation meter,

[0023]FIG. 2 is a schematic block diagram of part of the personalradiation meter shown in FIG. 1 illustrating operation of a means fortesting the radiation detector shown in FIG. 1,

[0024]FIG. 3 is a schematic block diagram of part of the personalradiation dosemeter shown in FIG. 1 showing a further arrangement fortesting the radiation detector, and

[0025]FIG. 4 is a schematic block diagram of an arrangement forgenerating an audible monitor signal in accordance with the detectedradiation by the radiation detector means.

[0026] Part of an electronic radiation dosemeter is illustrated in ablock diagram form shown in FIG. 1. In particular, FIG. 1 has beenprovided to illustrate an arrangement for forming a shield for thepersonal radiation dosemeter so as to provide protection from bothelectromagnetic interference and from low energy radiation particles.

[0027] In FIG. 1 a radiation detector 1, is shown to be disposed on afirst plane 2 of a printed circuit board 11. On a second plane 3 of theprinted circuit board, there is also shown further electronic components4, 5, disposed upon the second plane 3. Interposed between the first andsecond planes 2 and 3 of the printed circuit boards, is a ground orearth plane 6, which is fabricated from a suitable material such thatelectrical conductivity is provided along its length. Also shown in FIG.1 is an electrical via 8, for providing a means for electricallycoupling the radiation detector 1, to components on the second plane 3of the printed circuit board 11. As will be appreciated, the printedcircuit board 11 may be provided with a plurality of layers which may beelectrically connected using conventional technology known to thoseskilled in the art. Furthermore the physical arrangement of theelectronic components 4, 5, within an enclosure provided by the shieldmay be different to that shown in FIG. 1, and many other arrangementsmay be envisaged.

[0028] In operation an electronic personal radiation dosemeter detectsradioactive particles or high energy X-ray emissions using the radiationdetector 1, which serves to generate a signal indicative of the presenceof such a radiation particle. Such signals are thereafter fed to anelectronic measurement circuit which is made up from components 4, 5,disposed on the printed circuit board 3. Since the radiation detector 1,is arranged to detect the presence of radiation particles and highenergy photons, the radiation detector 1, must be shielded from otherextraneous radiation such as electromagnetic interference or otherelectrical signals. This protection is further enhanced by disposing theearth or ground plane 6 interposed between the first and second planes2, 3 of the circuit board.

[0029] Electronic personal radiation dosemeters are arranged to providean indication of an amount of radiation absorbed by the body. As such,it is necessary to provide the radiation detector 1 with an amount ofshielding sufficient to attenuate low energy radiation particles orX-ray photons so that radiation detected by the radiation detector is amore accurate representation of a total amount of energy received by thehuman body. Hence it is necessary to provide a shield, known as aradiological shield, to prevent low energy radioactive particles frombeing detected by radiation detector 1. The shielding should be arrangedto provide substantially 360° of shielding of the radiation detector 1.

[0030] As aforementioned, electronic radiation dosemeters are alsosensitive to electromagnetic interference. Examples of sources of suchelectromagnetic interference are stray electromagnetic fields frommonitors or from radar equipment which by its nature transmits highenergy electromagnetic pulses, or from mobile telecommunicationsequipment providing interference at microwave frequencies. It istherefore necessary to provide both electromagnetic and radiologicalscreening to an electronic radiation dosemeter. FIG. 1 provides anillustration of an arrangement of such a screen. In FIG. 1 a shield isprovided in a first part 10 and a second part 12. The first part 10, isarranged to provide a screened volume around the radiation detector 1.The shield first part 10, is arranged to be electrically coupled to theearth plane 6, at a plurality of conveniently located fixing points. Anexample of such fixing points is shown in FIG. 1 either side of shieldfirst part 10 at points 14, 16.

[0031] The second part of the screen 12, is also electrically coupled tothe earth plane 6, at points 18, 20. The second part of the shield 12,is arranged to provide a screened volume for the electronic components4, 5, of the electronic measurement circuit 7, disposed on the secondprinted circuit board 3. By electrically connecting the first and secondparts of the shield 10, 12, to the earth plane, the radiation detectorand electronic components 4, 5, are provided with an electromagneticshield in the form of a “Faraday cage”. The first and second parts ofthe shield 10, 12, are arranged to provide substantially 360° ofscreening such that there is no line of sight for any radiationparticles to the detector 1. As will be appreciated by the cogniscenti,the shield 10, 12, can also be fabricated in other arrangements.

[0032] By fabricating the first and second parts of the shield from amaterial which is both conductive and provides an appropriateattenuation of low energy radiation particles or low energy X-rayphotons, the shield 10, 12, may provide radiological shielding as wellas electromagnetic interference shielding. An example of such a materialmay be a metal such as tin. Tin may provide appropriate conductivity toeffect electromagnetic interference shielding, whilst having an atomicnumber which is sufficiently high to attenuate low energy radiationparticles before reaching the radiation detector 1. Another example ofsuch a material is zinc. A further example is to use an alloy such aspewter which combines copper and tin so that appropriate levels ofradiological shielding are effected whilst maintaining appropriateelectromagnetic interference shielding. Pewter has the further advantageof being mechanically workable so that the first and second parts of theshield 10, 12, may be formed, and has a melting point which is higherthan that of conventional solder. As such, the shield 10, 12, may besuitably shaped and formed in components and soldered using standardcircuit assembly techniques.

[0033] In use, electronic personal radiation dosemeters, are worn bypersonnel in potentially hazardous environments such as nuclear powerstations. As such an amount of radiation detected by such a dosemeterwill vary throughout daily use in accordance with an environment inwhich the personnel are present. For this reason, there may be periodsduring a day in which the personnel are exposed to little or noradiation. However, equally the personnel may be situated inenvironments where there are potentially hazardous levels of radiation.If a situation were to occur wherein the personal radiation dosemeterwere to malfunction in an environment in which little or no radiation ispresent, then personnel may be subjected to hazardous levels ofradiation without being alerted to this danger. To effect a remedy toundetected malfunctions of the personal radiation meter, a regular andfrequent automatic self-test facility is provided. Although testing ofthe electronic measurement circuit may be effected using a knownarrangement of coupling test signals into amplifiers and otherelectronic components of the measurement circuit, such test circuitswould not provide a suitable test for the integrity of the radiationdetector 1. An example of a radiation detector 1, is a photo sensitivedetector which is arranged to detect ionising radiation. Such radiationdetectors may also detect longer wavelength photons of light in thevisible or infra-red spectra and it is for this reason that theradiation detector 1, must be screened from ambient light for correctoperation.

[0034] An embodiment of the present invention illustrated in FIG. 2shows a means for providing a test for the integrity of the radiationdetector 1. FIG. 2 provides a schematic block diagram of part of theelectronic personal radiation dosemeter shown in FIG. 1 where parts alsoappearing in FIG. 1 bear identical numerical designations. In FIG. 2 alight emitting diode 22, is disposed on the second plane 3 of theprinted circuit board. In the first and second planes of the printedcircuit board and the earth plane 6, there is provided a through hole24, for communicating light generated by the light emitting diode 22,into the volume shielded by the first part of the shield 10. Alsocoupled to the light emitting diode 22, is a test circuit controller 26.The test circuit controller 26, is arranged to be as far as possibleelectrically isolated from other components of the electronicmeasurement circuit 7, such that there is no electrical coupling ofsignals from the test circuit controller 26, to the electronicmeasurement circuit 7.

[0035] In operation the test circuit controller 26, operates to controlthe light emitting diode 22, so as to provide a regular and frequenttest of the integrity of the radiation detector 1 in combination withthe electronic measurement circuit 7. The test circuit controller 26,operates to excite the light emitting diode 22, thereby generatingphotons of light which have a wavelength sufficient to cause detectionof such photons by the radiation detector 1 The photons pass through thethrough hole 24 into the volume in which the radiation detector 1 issituated. Photons may reach the radiation detector indirectly viareflection from the inside surface of the first part of the shield 10.An alternative arrangement is illustrated in FIG. 3 where parts alsoappearing in FIGS. 1 and 2 bear identical numerical designations. InFIG. 3 a fibre-optic 28, is provided to effect optical communication ofphotons generated by the light emitting diode 22, to the radiationdetector 1. As will be appreciated by those skilled in the art, othermeans may be provided to effect optical coupling of the light generatedby the light emitting diode 22 to the radiation detector 1.

[0036] Electronic personal radiation dosemeters are also provided with ameans for providing an audible monitor signal which alerts the carrierof the personal radiation dosemeter to the presence of radiation. Anexample of such an arrangement is an audio signal generator coupled tothe measurement circuit and arranged to generate an impulse of sound or‘chirp’ in accordance with radiation detected by the radiation detector1. In this way the frequency of the impulses or chirps is arranged to bein proportion with the current quantity of radiation detected by theradiation detector 1.

[0037] As is known to those skilled in the art, there are various typesof radiation particle. Each type of radiation particle may furthermorehave a substantial range of energy. As such, different radiationdetectors may be required to detect each of these types or energies ofradiation. For this reason, the personal radiation dosemeter may beprovided with a plurality of detectors, each of which is arranged todetect radiation particles with particular characteristics. The effectof a particle of radiation detected by the radiation detector for a lowenergy particle and radiation particle detected by the radiationdetector for high energy particles are required to be scaled inaccordance with difference in the harmful effects on the human body.After scaling, the personal radiation dosemeter must be arranged suchthat a repetition frequency of the audible monitoring signal is adjustedin accordance with the characteristics and quantity of radiationdetected by each radiation detector.

[0038] A schematic block diagram of an arrangement which serves togenerate such an audible monitoring signal is shown in FIG. 4. In FIG. 4three radiation detectors 1, 30, 32 are shown to be connected to anaudible monitor control circuit 34. Also connected to the audiblemonitor control circuit 34 are three data stores 36, 38 and 40.Connected to an output 42 of the audible monitor circuit controller 34is an accumulator 44. Connected to all output of the accumulator 44, isan audible signal generator 46. Connected to an output of the audiblesignal generator 46, is a loud speaker 48. The loud speaker 48, may forexample be a piezo buzzer. Although the audible signal generator 46shown in FIG. 4 has been illustrated with three radiation detectors 1,30, 32, it will be readily appreciated by those skilled in the art thata number of radiation detectors may be provided, each of which isarranged to detect a predetermined or predefined type of radiationparticle. Associated with each of the radiation detectors 1, 30, 32 isone of three data stores 36, 38, 40. Each of the data stores 36, 38, 40,is arranged to store a predetermined number representative of a typicaldose delivered to the human body by a radiation particle detected by thecorresponding radiation detector. In operation, the monitor circuitcontroller 34, operates to add the predetermined numbers stored in thedata stores 36, 38, 40, into the accumulator 44, consequent upon receiptof a signals from the corresponding radiation detector 1, 30, 32,indicative of a detected particle of radiation. Thus, if for exampleradiation detector 1, detects a radiation particle, then thepredetermined number stored in data store 36, is added to the runningtotal in accumulator 44. When the accumulated total contained inaccumulator 44, reaches a predefined numerical threshold, a signal isgenerated on conductor 45, representative of a predetermined amount ofradiation energy received by the radiation detector. The signal fed onconductor 45, from accumulator 44, is received by the audible signalgenerator 46, which operates to generate a sound impulse or chirp fed tothe loudspeaker 48, for audible conveyance of the detected quantity ofradiation. The predefined numerical threshold in combination with thepredetermined numerical values in the data stores 36, 38, 40 arearranged to generate a monitoring signal with a repetition frequency inproportion to the current dose rate from radiation particles received bythe radiation dosemeter. Once the monitor signal has been generated, therunning total in accumulator 44 is reduced by the predefined numericalthreshold, after which the aforementioned adding of the predeterminednumbers to the accumulator 44, continues. One way of effecting thisreduction is to set the aforementioned predetermined numbers so that thepredefined numerical threshold is the maximum value of the accumulator.The monitor signal may then be generated from the accumulator 44, bysimply an overflow indicator. However, as will be readily appreciated bythose skilled in the art, other means may be used in order to effect thetriggering of the monitor signal in accordance with a predeterminednumerical threshold and corresponding re-setting of the accumulator 44.

[0039] The monitor circuit 34 may be implemented using hardware logic.As such, a hardware implementation may provide a considerableimprovement in economy of power, over a use of a microprocessor. Use ofhardware to implement monitor circuit 34, instead of a microprocessorremoves a limitation on a repetition frequency at which audible signalscan be generated as a result of a microprocessor being powered up inaccordance with a predetermined duty cycle. As a result of a hardwareimplementation, the monitor circuit 34, may be operated continuously ina power efficient manner thereby obviating any requirement for operationin accordance with a duty cycle, and so removing any limit on therepetition frequency of the audible signals.

[0040] As will be appreciated by those skilled in the art, variousmodifications may be made to the embodiments hereinbefore describedwithout departing from the scope of the present invention. Inparticular, various other forms of shield may be constructed whilststill providing the combined radiological and electromagnetic shieldingeffects provided by the present invention. Furthermore, other means maybe provided for generating the photons of light used to test theradiation detector other than a light emitting diode. Other means mayalso be used for effecting operation of the audio monitor signal inaccordance with accumulating predetermined numbers representative of anaffect of radiation particles detected by a corresponding radiationdetector.

1. A personal radiation dosemeter comprising a radiation detector meanscoupled to an electronic measurement circuit arranged in combinationtherewith to generate signals representative of an amount of radiationdetected by said radiation detector, wherein said radiation detectormeans is provided with a light source optically coupled to saidradiation detector means and arranged to operate under control of a testcontrol circuit to generate light of a wavelength which may be detectedby said radiation detector, thereby providing in combination with saidelectronic measurement circuit an integrity test for said radiationdetector means.
 2. A personal radiation dosemeter as claimed in claim 1,wherein the light source is a light emitting diode.
 3. A personalradiation dosemeter as claimed in claims 1 or 2, wherein the opticalcoupling includes an optical fibre arranged to convey the light to saidradiation detector means.
 4. A personal radiation dosemeter as claimedin claims 1 or 2, wherein the optical coupling is effected by reflectionvia a surface of the shield.
 5. A personal radiation dosemeter asclaimed in any preceding claim, further including a screen which servesto shield said radiation detector from ambient light.
 6. A personalradiation dosemeter comprising a radiation detector means coupled to anelectronic measurement circuit and arranged in combination to generatesignals representative of an amount of radiation received by saidradiation detector means, wherein there is further provided a shieldarranged to be electrically coupled to an earth plane and tosubstantially surround a volume in which said radiation detector meansis disposed, said shield being fabricated from electrically conductivematerial so as to provide substantial electromagnetic screening, whichelectrically conductive material has a composition and densitysufficiently high to provide substantial radiological shielding, forsubstantially low energy radiation particles, said shield being therebyarranged to provide both electromagnetic and radiological screening. 7.A personal radiation dosemeter as claimed in claim 6, wherein saidelectrically conductive material is a metal.
 8. A personal radiationdosemeter as claimed in claim 7, wherein the metal is tin.
 9. A personalradiation dosemeter as claimed in claim 7, wherein said electricallyconductive material is an alloy.
 10. A personal radiation dosemeter asclaimed in claim 9, wherein the alloy is pewter.
 11. A personalradiation dosemeter for generating a monitor signal representative of aradiation dose rate, said radiation dosemeter comprising a radiationdetector means coupled to an electronic measurement circuit and arrangedin combination therewith to generate signals representative of an amountof radiation received by said radiation detector, wherein saidelectronic measurement circuit includes at least one data store, anaccumulator means and control circuit means, which control circuit meansis coupled to said radiation detector means and arranged to add apredetermined number stored in said data store to an accumulated totalstored in said accumulator in response to signals from said radiationdetector means, said control circuit being arranged to generate amonitor signal for each increment of said accumulated total by saidpredetermined numerical threshold, which monitor signal is fed to anaudio signal generator so as to provide an audible signal in accordancewith said increment, a repetition frequency of said audible signal beingthereby representative of said radiation dose rate.
 12. A personalradiation dosemeter as claimed in claim 11, further comprising at leastone other radiation detector means and at least one other data store,wherein said at least one other data store includes a furtherpredetermined number and said control circuit operates to add saidfurther predetermined number to said accumulator consequent upon receiptof signals from said at least one other radiation detector means.
 13. Apersonal radiation dosemeter as claimed in claim 12, wherein said firstand said further predetermined numbers are selected in combination withthe numerical threshold, so that the radiation dosemeter is arranged togenerate a monitor signal at a frequency which is arranged to provide anaudible indication determined by the relative harm caused by theradiation detected by the first and further radiation detector means.14. A personal radiation dosemeter as claimed in any of claims 10 to 12,wherein said predetermined numerical threshold is representative of amaximum count which is available to said accumulator, and said monitorsignal is generated from an overflow signal generated by theaccumulator.
 15. A method of generating a monitor signal representativeof a radiation dose rate, said method comprising the steps of: addingsaid predetermined number to an accumulated total upon receipt of afirst signal from a first radiation detector, comparing said accumulatedtotal with a predetermined numerical threshold and generating a monitorsignal if the accumulated total has changed since a last monitor signalwas generated by said predetermined numerical threshold.
 16. A method ofgenerating a monitor signal as claimed in claim 15, further includingthe steps of adding said further predetermined number to saidaccumulated total upon receipt of a further signal from a furtherradiation detector.
 17. A personal radiation dosemeter as hereinbeforedescribed with reference to FIGS. 1, 2, 3 and 4.